Note: Descriptions are shown in the official language in which they were submitted.
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Background of the In~ention
Field of the Invention
The present invention is in the field of mono-
lithic chip capacitors, especially chip capacitors adapted
to be connected directly to circuit boards as opposed to
capacitors having flexible leads extending therefrom.
The Prior Art
.
Curren~ electronic de~ices are making progressively
increasing use of chip capacitors because of their compactness
ænd inherent reliability.
In accordance with a conventional means for employing
such capacitors, the same are provided with two or more termina-
tion portions at their end edges and are mounted directly to a
substrate o~ alumina or epoxy-filled ~iberglass carrying conduc-
tive lands on the surf&ce thereof. Reflow solder connectîons are
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effected between the capacitors and the lands, the terminations
being in diract contact with the lands. The solder provides both
electrical and mechanical connections of the a.rticle to the sub-
strate.
In the course of soldering, and in many instances in
use, the device incorporating the circuit board and capa.citors
are subjected to thermal excursions.
Due to the differential coef~icients of expa.nsion of
the various connected materials and due further to ~he relative
fragility of the capacitors, particularly at the interface be-
tween the termination and the capacitor electrodes, a relatively
high incidence of capacitor failure has been experienced despite
the inherent reliability of the capacitor device. In order to
obviate such failure, attempts have been made to introduce a com-
pliant connection at the interface between the capacitor termina-
~ tion and circuit board, such that the differential shrinkage and
; expansion of the components will not exert undue stresses on the
fragile elements of the device but, rather, will be absorbed by~
flexure of the compliant connector. ~ ::
~ 20 While the utiliza:tion of such compliant connectors
:~ provides a capacitor having an extremely high degree of reliabil-:
: ity, the application of the compliance member, such as a tab, or~
the like has added a significa.nt element of cost increase by vir~-
tue of the presence of an additiona.l part, together with the
operations necessary to the applying of the part.
An additiona.l loss in reliability of the capacitor
is occasioned by the presence of voids in the area between the
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capacitor electrodes and oppo~ing termination. Such voids ha~e
been determined to be present in the capacitor body, in part as a
result of the fact that the electrodes between the dielectric
layers are o~ ~inite thickness. ~hus, when a unlt o~ compres-
si~e force is applied, utilizing conventional techniques, across
the opposed surfaces o~ the capacitor, grea*er pressures are
developed in the areas of greatest thickness, e.g., the central
area in registry with overlapped electrodes of opposite polar-
ity, than in the margina} areas wherein no overlapping of the
electrodes occurs. As a result of insufficient compression of
the marginal areas, weak spots or voids are present extending
~rom the end of the electrodes terminating short of the end of
the capacitor to the termination material at the end of the
capacitor, resulting in a capacitor susceptible to voltage
breakdown and/or degradat~on of insulation resistance, with
consequent changes in value.
Summary Or the Invention
The present invention may be summarized as directed
to an improved monolithic chip capacitor wherein the reflow
soldering step utillzed to connect the capacitor to the substrate
provides a degree of compliance sufficient to render the capacitor
highly resistant to damage under the influence of thermal shock,
whether such shock is experienced in the application of the
capac~tor to the substrate or in the subsequent utilization of
the device incorporating the capacitor.
More partlcularly, the invention is directed to an
impro~ed capacitor o~ the type described characterized in that
the bottom or substrate-adjacent surface thereof includes an
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integral depending support portion or portions which, when
disposed against the board, lift the lateral termination ends of
the capacitor incorporating the terminations a distance of from
about 5 to 10 mils or more from the surface of the substrate.
When the termina.tion portions of a-capacitor as
described are attached as by reflow soldering to the substrate,
there is formed a vertical solder bead which, by virtue of the
fact that the termination portions are spaced from the conductive
lands carried by the substrate, enables the solder bead to a.ct
as a compliant column or pillar, permitting compensation for
dissimilar thermal elongation cha.racteristics of the ca.pacitor
and the substrate to be absorbed by flexure of the column. The
: compliance of the elongated solder column formed by lifting the
termination aboYe the level of the substrate thus prevents ~`~
fracture, for exam~le, in the connection between the termination
portions and the end edges of the electrodes.
The specific means for separating the termination
: areas of the capacitor from the surface of the land may take
various forms. In accordance with a preferred embodiment, a
20 : central platform of ceramic may bQ integrally molded on the
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bottom surface of the capacitor, which platform is of a height
to lift the conductive terminations above the levels of the con- :
ductive lands on a substrate or circuit board when the plat~orm~
abuts the board~ The support means preferably is spaced inward- :
ly from the ends of the capacitor to which the termination ma~
terial is applied, a suffic~ient distance to assure that the re-
flow soldering results in the formation of a vold in the area
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between the conductive land and the support means, to avold com-
promising the compl~ant characteristics of the solder bead.
In accordance with a further embodiment of the inven-
tion, the support may comprise two or more ridges on the under-
sur~ace of the capacitor which function in the same manner as
the platform described above.
The invention will be illustratively described in
conjunction with a simple capacitor device having two termina-
tion portions, with multiple in~ernal electrodes.
In accordance with the preferred embodiment as des-
cribed above, the support means is formed by compressing the
marginal edges of the green ceramic of which the capacitor is
formed~ prior to firing~ to a greater degree than the central
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portion thereof, whereby there is derined below the bottom
surface of the capacitor a depending platform or ledge, the
noted practice having the additional advantage o~ compreseing
the marginal areas between electrodes and termination to exclude
the possibility of weak~spots or ~oids in such areas.
Accordingly, it is an ob3ect of the invention to
provide an improved monolithic chip capacitor whereby a reflow
solder-formed connection between the capacitor and a substrate
will inherently possess a degree of compliance sufficient to
preclude damage to the capacitor or to the connection between
the capacitor and substrate, due to dif~erentlal tbermal co-
efficients of expansion of the capacitor and the substrate~
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A further object of the invention is the provision
of an improved capacitor wherein the solder connection ~ormed
between the capacitor and substrate possesses a high degree of
compliance.
Still a ~urther object of the invention is the pro-
vision of a capaci~or of the type clescribed having a cost of
production not materially different from conventional capacitors
of the same type.
Still a further object of the invention is the pro-
vision of capacitors of the type described having increased
resistance to the for~ation of weak spots or voids in the margin
areas adjacent the capacitor terminations.
Still a further object of the invention is the pro-
vision of methods of manufacturing a capacitor of the type ~ ~
described. ~ -
To attain these objects and such further obaects
as may appear herein or be hereinafter pointed out, reference
is made to the accompanying drawings, forming a part hereof,
in which:
Figure 1 is a diagrammatic side elevational view o~
a capacitor in accordance with the invention applied to a sub-
s~rate;
Figure 2 is a perspective view of the under surfacè
of the capacitor of Figure l;
Figure 3 is a disgrammatic isometric view of a form-
ing die for fabrication of a capacitor in accordance with the
lnvention;
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Figure 4 is a diagrammatic isometric view of the
partially formed capacitor;
Figure 5 is a perspective view of a capacitor in ac-
cordance with a further embodiment of the invention,
Turning now to the drawings, there is shown in
Figure l a monolithic chip capacitor lO secured to an insulating
substrate ll. Typically, the substrate may comprise alumina or
epoxy filled glass fiber, both Of which materials have coefficie~ts
of thermal expansion differing from the thermal coefficient of
expansion of the capacitor. It should be recognized that the
capacitor itself is comprised Of a plurality of layers of
ceramic materials which are subject to fracture or separation
should the capacitor be exposed to undue compressive or expan-
sive stresses. Such stresses are present if, as is typical,
the conductive capacitor termination formed at the marginal ends
o~ the capacitor is secured directly against the conductive land
formed on the substrate, locking the capacitor against substan-
tial movement relative~to the substrate.
It will be appreciated that upon cooling of the com~
~;20 ponents after a solder bead has been formed, compressive or ex-
pansive forces are inevitably developed, resulting from the
differential thermal expansion factors. ~ ;
In accordance with the present invention, the effects
of such stresses and, to a degree, mechanical shocks are miti-
gated so as to avoid compromise of the capacitor or its connec-
tion to the substrate by the provision of support means~ e.g.
the platform 12 disclosed in the preferred embodiment of Figures 1
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to 4, or the ridges 13 a.s disclosed in the embodlment of
Figurè 5, which space the under surfa.ce 14 of the bottom of the
capacitOr a dista.nce D from the substrate 11 of from about 5
to 1~ or more mils from the upper sur~ace 15 o~ the substrate., to
ensure that a substantial expanse or vertical column 16 of solder
will exis~ between the termination portions 17, 17 of the capa-
citor and the substrate.
By the expedient of providing integral support means
on the ceramic body which, while unconnected to the substrate
lift the termination portions the aforesaid dista.nce above the
substrate, there is assured the provision of a solder connection
of su~ficient length to provide a relatively high compliance con~
nection between the ca.pacLtor and the conductive lands 18 of the ; ~ -
substrate such as to cause mechanically generated stresses to be ~:
absorbed or compensated within the solder body rather than be~
ing transmitted to the capacitor at sufficiently high levels to
cause capacitor ~ailure or cha.nge ln value.
~: It will be observed tha~ the compliance effect com~
; pensates both Ior relative extension and contraction of the
subs~rate and capacLtor.
Referring now to Figure 3, there is diagrammatically~disclosed a method of ~orming the capacitor of Figures 1 and 2.
As is conventional~ the capacitor is comprised of a series of ~ ;~
ceramic layers 1 defining the dielectric components of the
capa.citor and the encapsulation thereof.
Between the~layers L there are formed the electrodes
E, Et, which electrodes~ althOugh formed as by screening of
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metal onto an associated ceramic layer, are of finite thickness.
The electrodes of one polarity, e.g. the electrodes E, run from
marginal end 19 longitudinally, terminating at a position short
of the opposite marginal end 20 of the capa.citor. In similar
fashion, the electrodes E' of opposite polarity begin at a posi-
tion coextensive with the marginal end 20 and terminate short of
the opposite marginal end 19.
As will be readily recognized from the above described
essentially conventional construction, by virtue of the finite,
albeit small, thickness o~ the electrodes E, E', a series of
voids V may exist in the areas between the ma.rginal end 20 and
the electrodes E, and between the ma.rginai end 19 and the elec-
trodes E'. In order to provide both the support-spacer means 12
shown in the capacitor of Figures 1 and 2, and a.lso to elimln-
ate the voids which constitute areas of incipient capacitor
failure, there is provided a die member or die construction
which compresses the marginal portions M (being the portions
; where there is no overlap of electrodes of opposite polarity)
~: a greater degree than the central area wherein the opposite
~ ; 20 polarity electrodes overlap.
:~ . The die member, which has been diagrammatically il- :
lustrated may include a flat top pla.te 21 a.nd a ba.se plate 22
recessed as at 23 in the area in registry with the overlapping
portions of opposite polarity electrodes. I~hen the die member ;
is closed, the green ceramic in the marginal portions as above
defined will be compressed to a greater extent than, or at lea.st
to an extent equal to, the compression applied to the central area
resulting in a concomitant compression 0nd consequent elimination~
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of voids in registry with the marginal portions.
The de~ice a.t the same time forms the desired plat-
~rm 12 which~ as noted above, wil:L function to space the sub-
sequently formed end termination portions 17 a dista,nce above
the level o~ the substrate when the capacitor is soldered to a
circuit board or the like~ The method of forming the capacitor
is thus functionally distinguished from conventiona.l manufa.c-
turing processes wherein the entirety of the surfaces are com-
pressed between flat platens, resulting in greater pressures in
the area in reg~stry with the overlapping electrodes than in the
thinner ma.rg~nal areas.
There is illustrated in Figure 4 a capacitor sub-
assembly as removed from the die members 21, 22, illustrating
the effects of additional compression in the margin areas and
consequent elimination of the voids.
As will be understood by those skilled in the art,
the ceramic member illustrated in Figure 4 will be fired, fol- ~
lowing which termination portions 17 will be applied, provid- :
: ing means for connecting electrodes of opposite polarity into
the circuit.
In accordance with a, variation of the manufacturing
procedure, the die portion 22 employed as the means of forming
the under surface of the capacitor may incorporate a yieldablc
e-astomeric layer on the surface engaging the green ceramic~ :
The resilience of the ela.stomeric layer will automatically ef-
fect the desired additiona,l compression of the marginal portions,
the total thickness of which marginal portions~ by virtue of
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the absence of overlapping electrodes of opposite polarity, will,
after compression3 be less tha.n the thickness of the central
area whereat the electrodes overlap.
It will be understood that the die 22 may utilize
one or both of the expedients of recess and the resilient layer.
It is further possible, where the composite addition~
al thic~ness of the overlapping electrodes is sufficiently great,
to rely upon such addit~onal thickness in combination with a~
elastomeric die member to a.chieve the desired platform portion.
10As will be evident to those skilled in the art in
the light of the instant disclosure, numerous varia.tions may
be made in the concepts hereinabove set forth without depa.rt-
~ng from the spirit of the invention. For instance, the con-
figuration and number of ridges, steps or like support mea.ns
employed on the under surface o~ the capacitor are not critical
so long as the same satisfy the function of lifting the marginal
::- terminal edges o~ the capacitor to assure the existence of a sub~
:~ stantial span of solder between the termination ond the sub~
stra.te. m e support means are preferably spaced inwardly a:
20~ distance from the marginal edges to a.ssure that the solder does ;: :~
not ~low into the entire space between the land and the support:
portion, in which case there would be increased possibility of ::: :
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transmitting greater stresses to the capacitor.
Accordingly, the invention is to be broadly construed
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within the scope of tbe appended claims.
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